2.7.7Atoms, Elements and Compounds
Prepare an Insoluble Salt — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 26 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5The Precipitation Reaction
Two soluble salts react, and the insoluble salt separates out as a solid.
1
Measure equal volumes of the two soluble salt solutions and mix them together in a beaker, stirring to ensure reaction.
2
A solid precipitate forms immediately because one of the products is insoluble in water.
3
Filter the mixture so the precipitate is retained on the filter paper and the solution passes through.
4
Wash the precipitate with distilled water to remove traces of the other solutions, then leave it to dry.
Section 2 of 5Choosing the Reactants
The two reactants must both be soluble, and their products must include one insoluble salt.
Soluble reactants
- Soluble reactants Both starting salts must dissolve in water so that their ions can mix freely.
Insoluble product
- Insoluble product The salt to be made must be insoluble, so it separates as the precipitate.
Nitrates as sources
- Nitrates as sources All nitrates are soluble, so silver nitrate or lead(II) nitrate are often used as the source of silver or lead ions.
Double decomposition
- Double decomposition The reaction type AB + CD → AD + CB is also called double decomposition, since ions swap partners.
| Insoluble salt | Typical reactants |
|---|---|
| Silver chloride | Silver nitrate + sodium chloride |
| Lead(II) iodide | Lead(II) nitrate + potassium iodide |
| Barium sulfate | Barium chloride + sodium sulfate |
Section 3 of 5Ensuring a Good Yield
Ensuring a Good Yield
Slight excess
- Slight excess One solution is added in slight excess so that the other ion is fully precipitated.
Measured volumes
- Measured volumes Using fixed volumes of each solution allows the amount of precipitate to be compared between experiments.
Washing
- Washing Washing removes soluble contaminants, which would otherwise remain in the dried precipitate.
Drying
- Drying The precipitate is dried at a low temperature to avoid changing the solid before it is weighed or used.
Section 4 of 5Worked Example
Question Write the balanced symbol equation, state symbols and ionic equation for the precipitation of silver chloride from silver nitrate and sodium chloride solutions.
- Symbol equationAgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
- The sodium and nitrate ions are spectators, so they are removed from the ionic equation.
- Ionic equationAg⁺(aq) + Cl⁻(aq) → AgCl(s).
Answer
- AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq);
- ionic equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Section 5 of 5Exam Checklist & Tips
2.7.7 – Key Points for the Exam
- Precipitate An insoluble solid formed when two solutions react together.
- Precipitation A reaction in which an insoluble solid is formed from solutions.
- Spectator ion An ion that is present in the same form on both sides of an equation and takes no part in the reaction.
- Filtration Separating a solid from a liquid by passing the mixture through filter paper.
- Double decomposition A reaction in which two ionic compounds swap ions to form two new compounds.
Common Mistakes
- Trying to make an insoluble salt by reacting an insoluble base with an acid;
- precipitation needs two soluble reactants.
- Forgetting to wash the precipitate, which leaves soluble contaminants in the solid.
- Filtering off the solution and keeping it as the product, when the precipitate is the required salt.
- Writing the state symbol (aq) for the precipitate instead of (s).
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